I'm working on a JD 755 compact tractor with a Yanmar 3TNA72UJ engine. 3000+ hrs. IP#'s 719621-51100 T606 7H21

When cold, it won't rev up and surges, almost stalling. Anything below 1/2 throttle and it stalls. It will blow black smoke for a split second every time the rpms surge. After it warms up some, it smooths out and revs up, but if you try to let it idle it does the surging thing again. If I go from idle to full throttle it runs exactly the same, surging at idle rpms for about 15 seconds, then it revs up and will go all the way to redline. It lacks power... trying to drive it or engaging the mower deck stalls it.

What I've done:
Replaced fuel filter. Filled filter with dieselkleen. It has an electric lift pump and it is getting plenty of fuel to the IP. I bleed it at the IP inlet.
Checked air filter.. I had the whole assy removed at the intake and it still ran the same.
Tried bleeding the injectors. I created a leak at one of the return lines when I disturbed it. I'm 99% sure it wasn't leaking before I messed with it, so I don't think it has anything to do with it.

I'm guessing it needs an injection pump, but I don't have a lot of experience with diesels, so I am asking for your input. Thank you for any help you can offer.

 

Today I removed the injectors, spun the line assy around and hooked the injectors back up so I could check for spray. All three were spraying what I thought looked like a decent pattern. Put it back together and it still rann like crap.

 

it would seem it has to be a problem in the IP, and if you have it rebuilt, you may as well swap the injectors with it so they're all new and happy together - even though it doesn't have all that much use behind it.

other possibilities are fuel starvation, air intrusion on the lift pump suction line, or similar fuel supply problems - but you seem to have ruled those out.
does it have a water seperator unit in the fuel line, or is there a chance you're getting some water into the IP and screwing around with that - i don't know exactly what it would do, but it wouldn't be good.

 

could be a broken spring on the gov, incorrect pop pressure on the injectors.

take the injectors out and fire them in the air. also pop test them.

if both of those don't do it, have the ip rebuilt..

did you change anything before this started occurring?

 

Did someone mess with it trying to get more power? It sounds like a combination of a missing/misadjusted rack damper and torque screw has been turned out.
What pump does that have?

 

I happen to also have a JD 855 to compare with and noticed that the 755 IP does not have the safety wire to keep someone from adjusting it, so there is no telling what has been messed with. The #'s on the pump: 719621-51100 T606 7H21

 

I had another thought on this last night- Make sure the return system flows fuel. If it doesn't the engine could be getting extra fuel and the pump is trying to compensate.

 

Another thought- Is the electric fuel pump original or an add-on? I just got a Yanmar tractor, and learning about them it seems the majority of them use a gravity fed injection pump without a pump return, just the injector returns.
If it doesn't have a return from the injection pump itself, I'd try running it without the electric pump in the loop.